The assembly has refused to withdraw its allegations that costs were running out of control.
This latest twist in the story of the assembly comes three months after Rogers returned to the much-delayed scheme after being sacked for running over budget. The architect is now operating under the supervision of project manager Taylor Woodrow.
The Welsh assembly asked its audit committee in January to investigate why an adjudicator had decided that it should pay Rogers £448,000 in adjudication fees. The committee then asked Shortridge why the assembly had been so confident it was going to win the adjudication.
Shortridge said: "The main problem throughout this project is that the original estimate submitted by Richard Rogers Partnership was too low. If the true costs had been known at the time, its entry would have been rejected from the competition."
The assembly has refused to withdraw its allegations that costs were out of control
Only civil servants were invited to give evidence to the investigation. Rogers had to tell committee chairman Dafydd Wigley by letter that Shortridge's statement was "wholly unacceptable".
In his letter, Rogers told the committee that the adjudicator had said the assembly was "not entitled to a declaration that Rogers misrepresented the proper cost".
Shortridge responded that the adjudicator had also said that the assembly "appears to be justified in alleging that Rogers had underestimated the cost".